Battery Pack: Rough Range Calculations

Once again, I’m going to go back to my napkin calculations to figure how big a pack I want for my target range of 75 miles or so. The simplest way to get to that is to look at the two documented bikes out there, grab a quick kW/mile figure from them, and you should be close. Here are the Brammo Empulse specs:

Battery Pack Capacity

9.31 kWh (nominal), 10.2 kWh (max)

Driving Range

City: 128 miles / 206 km

Highway: 58 miles/ 93 km

Combined: 80 miles / 129 km

The Zero SR:

Max capacity (wo/w Power Tank)

12.5 kWh

15.3 kWh

Nominal capacity

11.0 kWh

13.5 kW

Range

ZERO SRzf12.5

ZERO SRZF12.5 +Power Tank

City

151 miles (243 km)

185 miles (298 km)

Highway, 55 mph (89 km/h)

94 miles (151 km)

115 miles (185 km)

» Combined

115 miles (185 km)

141 miles (227 km)

Highway, 70 mph (113 km/h)

77 miles (124 km)

94 miles (151 km)

» Combined

102 miles (164 km)

125 miles (201 km)

The Energica Ego, just for comparison to a bike more similar in performance, has an 11.7kwh pack and the range numbers are 190 km @ 60 km/h, 150 km @ 80 km/h and 100 km @ 100 km/h. That translates to 118 miles @ 37 mph, 93 miles @ 50mph and 62 miles @ 62 mph

So it would seem the goal of a 75 mile range isn’t too far out of the ballpark, especially considering we’re targeting a lighter bike.

Rough napkin calculations? Give me a 10 – 11kWh pack and I’ll be happy. But let’s get legit and get some kWh/mile numbers from this.

The Brammo Empulse, at an average of 88 miles with a 10.3kWh pack is going at .117 kWh/mile.

The Zero SR, at an average of 108.5 miles with the base 11 kWh pack is at .10 kWh/mile. The SR with the Power tank is 133 miles with at 13.5 kWh pack, so also .10 kWh/mile.

And the Energica Ego, at an average of 91 miles with 11.7 kWh, you get .129 kWh/mile. (I’m a little skeptical of that data point simply because the range numbers are quoted at such low speeds.)

Conclusion? If we want 75 miles (average), we need something in the neighborhood of a 9 kWh pack, minimum, but more healthy would be 11 or so. Isn’t that what I said in the first place?

This is all good for a seat of the pants, drinkin’ ’round the fire idea, but stay tuned. We have some legit modeling going on behind the scenes, which should be fun to see how closely our napkin hit the mark.

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4 responses to “Battery Pack: Rough Range Calculations”

You also have to consider how much of your battery capacity you want to “save”. I have a Zero SR with Power Tank. Average capacity is 13.01 kwh. But if you run the battery all the way down and then multiply the wh/m times the miles you rode you find you have roughly 10kwh available to you out of that 13.01 kwh pack. This is why you have to be careful comparing battery packs from different companies. You can’t tell how much capacity you actually have available until you actually run the battery all the way down.

Also: My 60 MPH wh/mile figure is about 102 in the summer and can be over 110 in colder temps. At 70 MPH the figure is rarely under 120 and generally in the 130-140 range.

Range calculations are, at best, a shot in the dark – for many of the reasons you point out, and a few more. The numbers I pulled are averages of averages, and while engineers, I’m sure, are striving for an accurate and correct model, I am more than satisfied with a ballpark that gets me close to my targets.

I think I said this on one of the forums, but my priorities are quickness, top speed, and, far below that, range. Even top speed is a target I’m willing to give in on, considering 120mph is about as fast as I ever want to go on a bike. It’s all about how quickly I get there.

All that said, the modeling we’re working on is going to have a very precise look at a few key batteries based on some very in-depth analysis. Stay tuned. 😉

Also – on actual kWh available, your bike is limited by the BMS and controller, based on Zero’s determination of the “safe” limits of the chemistry. Most of the chemistries I’m looking at have very similar low-voltage cutoff safety levels, so it actually kind of comes out in the wash, at a real-world level.

But yeah, you’re not going to pull all the kWh out of your pack for more than a few times.